Animal models have been essential in allowing progress in our understanding of other subjectively- defined symptoms such as anxiety and depression, but currently researchers have no widely accepted animal model for the study of fatigue. This grant will develop a new animal model of fatigue based on interventions expected to decrease circadian amplitude. The long-term goal is to understand the role of the circadian system in the physiological and neural basis for fatigue. The objective in the current application is to validate an animal model of fatigue, building on our preliminary studies that demonstrate selective behavioral effects of chronic proinflammatory cytokines. The central hypothesis is that reduction of circadian outputs induces behavioral changes that can be validated as indicating fatigue in rodents. Our rationale for pursuing this line of research is the recognition that progress in understanding the neurobiology of fatigue will be facilitated by detailing the role of the circadian system as an important modulator of the neural and physiological pathways stimulating arousal and activity. The expected outcomes will be a novel animal model of fatigue. Specific Aim 1: Develop an animal model of fatigue based on chronic low-grade inflammation. Experiments will test the working hypothesis, based on preliminary data, that chronic pro-inflammatory cytokine IL-12 administration will dramatically decrease more effortful tunnel burrowing and wheel-running activity but will have little effect on general locomotor activity. To characterize this model as one related to aging, this experiment will demonstrate differential effects of chronic pro-inflammatory cytokines on fatigue depending on the age or gender of the animal. Specific Aim 2: Determine if our animal model of fatigue involves disruption of circadian clock output. Experiments will test the working hypothesis that treatment with pro-inflammatory cytokines will lead to circadian disruption measured in mice housed under constant darkness, interacting with the age and gender of the animal. To assess this treatment some experiments will use mice with the Per2Luc transgene and will assess circadian desynchrony by in vitro measures of bioluminescence. What may make the proposed project particularly innovative is the application of the PI's expertise in chronobiology to the question of the neural basis of fatigue. This contribution is significant because it will open the study of fatigue to laboratory animal researchers and chronobiologists, enabling studies of fatigue at levels such as mapping neural circuits, modulating neurotransmitters, or targeting genetic manipulations. 1